The long-term goals of this research are to understand the mechanisms responsible for the establishment and maintenance of tissue-specific patterns of transcription in mammalian cells. Using cultured hepatoma cells and hepatoma hybrids as a model system, trans-acting genetic loci that regulate expression of sets of liver genes have been defined. To determine the mechanisms by which these tissue-specific extinguisher (TSE) loci function and to assess their roles in cellular development, three main areas of research will be pursued. First, microcell hybrids retaining specific donor chromosomes will be used to complete the genetic analysis of extinction, the process by which tissue-specific gene activity is repressed in intertypic hybrid cells. Second, cDNA and genomic clones from two human TSE loci will be isolated and characterized. Third, the functions of TSE gene products will be studied by in vitro mutagenesis of cloned sequences and gene transfer. In particular, the potential roles of these products in regulating transcription factor expression will be assessed. These studies should help define regulatory networks involved in eukaryotic gene control, and may provide insights into perturbation of those controls that underlie abnormal development and neoplasia.